In the motor sport of drag racing, two cars line up side by side on a starting line. A series of starting lights mounted on a "Christmas tree" sequentially count down until a green light appears which signals the start of the race. The racers take off from the starting line and race each other to the end of the track which consists of a straight two lane road typically up to a quarter mile in length. The cars are timed by an electronic unit that times how long it takes for each car to run the length of the race course from the starting line to the finish line. The amount of time required to traverse the race track is called the "Elapsed Time" or more commonly, the "ET."
In some classes of drag racing, notably "Bracket Racing" or "Super Class Racing", the driver, the race track, or the race sanctioning association selects the ET that the car should run. This is known in racing as the "Dial In." The object of a racer is to get to the finish line ahead of his opponent without going quicker that his "Dial In." If the racer goes quicker than his "Dial In" and his opponent does not, or if both racers go quicker than their "Dial Ins", the racer who goes furthest under his "Dial In" gets disqualified and his opponent wins the race.
The purpose of this type of racing is to minimize the cost of campaigning a race car. In this type of racing, a slow car can race a fast car by having the racetrack "handicap" the fast car. "Handicapping" allows the slower car to start first by an amount of time that is equal to the difference between the "Dial Ins" of the two cars (the handicap). In theory, if both cars leave the starting line exactly when their respective green Christmas tree light turns on, and they run perfectly on their "Dial In", they should cross the finish line at the same time.
The other form of racing using this same method is the "Super Classes." In these classes, both cars are assigned the same "Dial In" and therefore, both cars leave the starting line at the same time. They race each other and try to finish first without going quicker than the assigned "Dial In."
Again, the purpose is to minimize the cost of racing.
In the "Super Classes" where the "Dial In" is assigned by the track or the race sanctioning body, the race car engines must produce excess power so that they can run quicker than the "Dial In." This is so that if track or weather conditions cause a car to run slower than normal, the car will still have enough power to run at least as quick as the "Dial In."
This creates a situation where the car will always run too quickly under normal conditions and so it must be slowed down. Devices known as "throttle stops" were created to selectively limit the power of race car engines. By setting the "throttle stop", the engine power level can be adjusted up or down to allow the car to run at exactly the "Dial In" elapsed time regardless of the track or weather conditions. An additional benefit of using a "throttle stop" is that it can be turned on and off (changed from full power to "limited" or "throttle stopped" power) as the car goes down the track. This usually results in a car having a higher speed at the end of the track than would normally be expected for a car that runs the selected ET or Dial In. Thus, a faster car chases a slower car which is an advantage because the faster car driver can judge how fast he is closing in on the slower car and he can also judge when he will cross the finish line. The slower car driver must continually look over his shoulder to see the faster car coming up behind him and then he must turn around to look at the finish line. Because of the above mentioned advantages of using a "throttle stop", they are widely used and the art is well known.
There are two basic types of "throttle stop".
One type is a "linkage style" (see, for example, Dedenbear Products, Inc. catalog, volume 5, page 18 model TS-10) which is a collapsible link within the throttle linkage between the gas pedal and the fuel metering device (carburetor or fuel injector). The throttle linkage length changes and therefore the butterflies on the fuel metering device close and limit the amount of air flow (engine power). This style is inexpensive and easily adaptable to many types of fuel metering devices, however, its disadvantage is that most racing fuel metering devices do not perform well under partial throttle conditions and therefore the cars performance becomes erratic.
The second and currently preferred type of throttle stop is the "baseplate" style. In this throttle stop, a second set of butterflies located underneath the fuel metering device controls the total air/fuel mixture flow after the fuel has been injected into the airstream by the fuel metering device. The advantage of this type of throttle stop is that at all times during a race, the fuel metering device runs at its optimum condition of wide open throttle so that the fuel metering and therefore the car performance stays very consistent. This style of throttle stop was created in 1987 by Dedenbear Products, Inc. and has been used to win many World drag racing championships (see Dedenbear Products, Inc. catalog, volume 5, pages 15-17 models TS-1 and TS-5).
Racing has progressed and the trend currently is to build large displacement, high horsepower engines in order to create very high speeds at the finish of the race.
Huge fuel metering device air flow rates are required to produce this high horsepower and an effect that is starting to become important is the impediment to air flow created by the second set of butterflies located in the throttle stop.